20120730

From the previous presentation, we thought about going backwards in time to determine the approximate age of the universe, and found that the space between everything approaches zero around 14 billion years ago--the big bang. (Video link: "The Big Bang.")

As with Brad Pitt's character aging backwards in The Curious Case of Benjamin Button (Warner Bros., 2008), we will go through the early stages of the big bang in reverse chronological order, at each step moving to the next older, hotter, and denser phase. Our current understanding of physics does not extend all the way back to time zero, so we are only going to cover four key events in the early stages of the big bang. You are later welcome to run these events forwards in time, as in your textbook.

So let's go back to the beginning!

This is a busy graphic, which we'll show again at the end of this presentation, but from right-to-left it shows key events in reverse chronological order, from now to 14 billion years ago; and from left-to-right these events in proper chronological order from 14 billion years ago to today.

First, the first stars.

Running the clock backwards from the present day--that is, looking out further and further away--we can see galaxies with less and less metallicity, and eventually a time/place where there were no galaxies. This is a time when the first-generation stars were born out of primeval hydrogen. (Video link: "WMAP’s Portrait of the Early Universe.")

Observations of these hyper-massive stars forming from raw hydrogen show them exciting the hydrogen atoms around them (as discussed in a previous presentation), creating gigantic emission nebulae. The process of exciting hydrogen electrons causes the electrons to be stripped off, ionizing the hydrogen.

Second, the first photons.

Looking further out away from the first-generation stars means looking further back in time, to before the the first-generation stars were born, where there is only hydrogen, and no stars--this is the dark age of the universe. (Video link: "WMAP’s Portrait of the Early Universe.")

Looking even further away and further back in time, we run into a figurative wall of light in every direction: this is the cosmic microwave background, and while this radiation is not the first photons ever created, it is the first and oldest photons that we are allowed to see. (Video link: "WMAP’s Portrait of the Early Universe.")

These intense photons are detectable today as a faint cosmic microwave background, which can be detected with special radio telescopes, and also televisions with old standard-definition rabbit ears--on an unused channel, a portion of the static "snow" that you can see is produced by the CMB signal.

So what would suddenly release this wall of photons, while making older photons that existed before this time be unobservable to us today?

This is a much hotter and denser universe than the dark ages, so hot that positively charged protons and negatively electrons are separated from each other, and are too energetic to be allowed to join up to make neutral atoms. Think of a mosh pit of protons and electrons constantly bashing into each other. Any photons in this hot, dense universe would be continuously kicked around and scattered, so light from this time and earlier could not have survived intact be observable today. (Video link: "Journey to the Centre of the Sun.")

But as the universe gradually cools and expands, protons and electrons calm down such that they can join up with each other, and as soon as they are allowed to do so, they all do so. This event where all protons and electrons suddenly form neutral atoms is recombination. Back to our mosh pit analogy, when the protons and electrons calm down, pairing off to slow dance with each other, then space on the floor drastically opens up. Photons at this point are now free to travel without being kicked around and scattered, and the cosmic microwave background are the first, oldest photons that started out from recombination. (Video link: "Journey to the Centre of the Sun.")

Third, the first fusion...even before the first-generation stars.

Going back in time even before recombination to a even hotter and denser time, the entire universe resembles the core of a star, where temperatures and pressures are high enough for protons to collide with each other and fuse. This is the nucleosynthesis phase of the early big bang.

For a brief time in this early universe, hydrogen fused into helium as in the cores of main-sequence stars, but can also fuse into deuterium (an unstable form of hydrogen) as well as lithium. What makes this early fusion in the universe unique is that deuterium and lithium cannot survive the high temperatures in the core of main-sequence stars--only helium survives--but the expansion and cooling of the universe shut off fusion such that deuterium and lithium from nucleosynthesis did survive to the present day. All the deuterium (found as "heavy water" in certain springs on Earth) and all the lithium (used in batteries for hybrid cars, computers and smartphones) that is found today can only have come from this early universe fusion.

Fourth, the first matter...stuff that makes up you, me, and the rest of the universe.

This is the farthest back in time we will go in our presentation, where the universe is intensely hot and dense. These are conditions we can reproduce using particle colliders today, and to investigate earlier times in the big bang merely requires building higher energy particle colliders (and money). The universe at this stage is so energetic that as in modern-day particle colliders, mass and energy are readily interchangeable--given enough confined energy, you can produce matter...and antimatter.

For this process of pair production, if energy creates a proton, it must also create the proton's "evil twin," an antiproton as well. Annihilation is the process where if a proton and its antiproton meet up with each other, they will be destroyed and converted back into energy. Everything that makes up matter has an antimatter "evil twin," and the very early universe continuously converted energy into equal amounts of matter and antimatter, and vice versa.

This presents a puzzle, as everything that we observe in the universe today is made of matter, but from pair production matter must be created with an equal amount of antimatter--so perhaps, somewhere else in the universe there is an equal amount of antimatter waiting to partner up with and subsequently annihilate us. Have you ever seen Seven Brides for Seven Brothers (MGM, 1954)?

What would happen if there was a sequel to this move...titled Seven Brides for Eight Brothers?

This is the hypothesis for why the universe is only made of matter today. Somehow instead of being produced in equal amounts, if there was slightly more matter than antimatter, then there would be something left over after all the antimatter produced is annihilated with an equal amount of matter. We today would then be the eighth unmatched brother, which makes us either fortunate or forlorn, depending on your perspective. In particle colliders equal amounts of matter and antimatter always seem to generated from pair production, but experiments being carried out today are searching for indications that there are certain processes that produce just a smidgen more matter than antimatter.

We've run the clock backwards, watching the first stars, first photons, first fusion, and first matter stages of the early big bang occur in reverse chronological order. We haven't gone all the way back to time zero, and there are some interesting events observed and conjecture that occurred even before these four stages that we will not cover in this course.

So concentrate on being able to describe what happened in each of these four stages, and the evidence that is either observed or recreated for each of these four stages.

20120729

Consider the big bang, where the universe began from an infinitely dense, high energy state, expanding and cooling off, while stars form in galaxies, which are moving apart from each other. "Cool story, bro." (Video link: "The Big Bang.")

The story of the big bang is a great story--it purports to explain everything that is today with a compelling narrative--a "just-so" story. For example, Rudyard Kipling's short story "The Elephant's Child" from his Just So Stories for Little Children explains that the crocodile gave the elephant a long trunk by stretching it from an originally shorter snout, and that is why all elephants have long trunks today. A tall tale, indeed, because there is no convincing evidence this could have ever happened...

...or could it have? (Don't worry, the baby elephant helped to rescue the mother elephant from the crocodile.) While this photograph of a single remarkable event shouldn't be taken as evidence that Rudyard Kipling's just-so story is true, we are going to explore various pieces of evidence that taken together support the big bang just-so story, elevating it from a compelling narrative taken only for its entertainment value to a compelling theory based on observable facts.

This presentation will explore three important pieces of evidence that provide clues for the existence of the big bang. A subsequent presentation will delve into key events in the early universe, when the first matter, atoms, photons, and stars were created to become what they are today.

First, the finite speed of light. It turns out that we have a time-machine of sorts to actually see the universe as it was in the past.

Consider Deneb, the farthest star in the night sky that can be seen from San Luis Obispo, CA with the naked eye. It is 1,400 light years away, and from the definition of the light year, light from Deneb takes 1,400 years to travel to us. This means that when we look at Deneb in the night sky tonight (season permitting), we are seeing Deneb not as it is now, but as it was in the past, 1,400 years ago.

In a dark sky location, you can observe with the naked eye a globular star cluster just outside the plane of the Milky Way disk, out in the halo, as a faint smudge: Messier 13, or the Great Globular Cluster in Hercules. The stars in this cluster are 25,100 light years away, so light from M13 takes 25,100 years to travel to us. This means that when we look at M13 in the night sky tonight (again, season permitting), we are seeing the stars in that cluster not as they are now, but as they were in the past 25,100 years ago.

The farthest object that you can see in a dark sky location with the naked eye is the Andromeda Galaxy (Messier 31), our nearest large neighbor galaxy, 25.4 million light years away. When we look at the Andromeda Galaxy in the night sky tonight (season permitting), we are seeing the stars that galaxy not as they are now, but as they were in the past 25.4 million years ago.

Because of the finite speed of light, looking at objects further and further out away from us lets us see those objects further and further back in the past. Using telescopes with lots of light-gathering power allows us to see very faint, very distant objects as they were a long, long time ago. Here the Hubble Space Telescope was used to collect light from one region of the sky for many months, and observed extremely distant galaxies as they were an extremely long time ago. So we do not need to merely conjecture how galaxies changed over time to become as they are today, we can directly see how galaxies have changed over time to become as they are today, by observing them at different distances from us.

Today, distant galaxies have __________ metals than nearby galaxies. (Assume that all galaxies starting forming at the same time, nearly 14 billion years ago.)
(A) less.
(B) the same amount of.
(C) more.
(D) (Unsure/lost/guessing/help!)

As observed from Earth, distant galaxies appear to have __________ metals than nearby galaxies. (Assume that all galaxies starting forming at the same time, nearly 14 billion years ago.)
(A) less.
(B) the same amount of.
(C) more.
(D) (Unsure/lost/guessing/help!)

What will we see if we keeping looking further and further out, further and further back in time?

Second, Olbers' paradox, which we'll treat as Olbers' question, as it is a question with an actual answer.

Suppose you were lost in a forest, with no marked trail, and having no directional clues (sunlight, moss on trees) nor navigational devices (compass, GPS tracker). And it will be dark soon. And there are werewolves. So it is important to be able to get out of this forest in the most expedient manner. If you look around in all directions, the quickest way out is to look for gaps between trees. If in a certain direction you see a clearing in the gaps between trees, then head in that direction, that is the edge of the forest. If in a certain direction there are more trees beyond in the gaps between trees, then the forest continues further on in that direction. If in every direction there are more trees beyond in the gaps between trees, then you're doomed, as the forest would be infinitely large...or at least, the forest is large enough that the edges cannot be readily seen. (Video link: "100806-1180532.")

Heinrich Olbers asked this same question of the universe. Is there an edge to the universe? Or does it continue on forever, or at least, is large enough that the edges cannot be readily seen? Let's answer this with the view from the Hubble Space Telescope, considering that every galaxy in the universe that can ever be seen in these directions will show up in this extremely long time-exposure. Since we don't keep seeing more galaxies beyond in the gaps between galaxies, Olbers' question is answered: the universe is not infinite; it is actually finite, and has an edge in all directions. (Video link: "Pan across the Hubble's Advance Camera for Surveys Ultra Deep Field.")

However, remember that the farther we look out in space, the farther back in time we are looking. Here is a three-dimensional representation of the galaxies observed by the Hubble Space Telescope. As we look further out in space, we run out of galaxies to see, which actually does not mean that we've reached an edge in space--we are going back in time, and have run out of time (or more precisely, a time before galaxies existed). So the edge we see is an edge in time, not space, and the actual answer to Olbers' question is that universe is not infinitely old, but has a finite age. (Video link: "Hubble Ultra Deep Field 3-D Fly-Thru.")

So if the universe has not been around forever, for how long has it been around?

Third, the Hubble law.

This is the actual data for nearby galaxies taken by Edwin Hubble, looking at their blueshifts and redshifts, or their velocities towards us or away from us (as discussed in a previous presentation). Note that the nearest galaxies are blueshifted, which means they are headed towards us, due to gravitational interaction (which is okay, if you would like the Milky Way to continue to have spiral arms), but as you look further out, galaxies are more and more redshifted, which means they are moving faster and faster away from us. This is the Hubble law, which suggests some sort of cosmic HateradeTM, as except for our immediate vicinity, every other galaxy in the universe is fleeing away from the Milky Way.

The interpretation for the Hubble law is not that all galaxies in the universe are running away from the Milky Way, but that the universe is expanding, and in a strange sort of manner. In this animation note that the size of the galaxies remain constant (due to gravitational forces within), but that the space between galaxies is increasing. Note that this is different than merely continuously "zooming in." Why the space between galaxies increases is not readily explainable, but the expansion of space between galaxies is consistent with the observations behind Hubble's law. (Video link: "Continued Consistent Expansion.")

If the space between galaxies is expanding today, let's think about going backwards in time, and considering how long ago was the space between galaxies zero? This is the Hubble age of the universe, and we find that at the current rate space between galaxies is expanding, there was no space between galaxies approximately 14 billion years ago (give or take), giving us an estimate for the finite age of the universe. (Video link: "The Big Bang.")

Keep in mind these key pieces of evidence that support the big bang--not a "just-so" story, but a "because-so" theory.